Active matrix (AM) and other types of LCDs are increasingly being used as display devices in aircraft. To accommodate low level night operation and high ambient sunlight conditions, an extremely large backlight dimming range is necessary. A desired luminance range may be as large 0.03 FtL (Foot-lambarts) to 200 FtL (6667:1) over the display field of view. This is a much larger range than previously exhibited by typical hot cathode or cold cathode fluorescent lamps. Previous backlight designs for backlighting LCDs have exhibited a limited luminance range of only approximately 2000:1 for hot cathode lamps and 500:1 for cold cathode lamps. Conventional backlight designs cannot typically achieve the low end of the luminance range without the fluorescent lamp extinguishing or flickering.
Typical AM-LCD backlight and display systems in military applications include an optical filter to enhance display performance while pilots are utilizing night vision imaging systems (NVIS). However, inclusion of these films or devices in the optical stack decreases the maximum luminance of the display in day mode operations. Since the difficulty in providing the extremely large dimming range occurs primarily at very low luminance levels, a backlight design capable of achieving a very low luminance uniform output without significantly affecting the maximum luminance level would be a significant improvement.